This invention relates to a biocompatible titanium base alloy characterized by high strength, low modulus and ductility, and to a method for the preparation of said alloy. The alloy of the invention is particularly suitable for the manufacture of prostheses and the invention also is concerned with a prosthesis made from the alloy.
Titanium base alloys for a variety of structural applications are known in the art and there are numerous patent and literature references disclosing a wide range of alloying elements which are used to provide alloys having desired characteristics, such as increased tensile strength and ductility. Generally, titanium and its alloys may exist in one or a mixture of two basic crystalline structures: the alpha phase, which is a hexagonal close-packed (HCP) structure, and the beta phase which is a body-centered cubic (BCC) structure. The transition temperature from the alpha to the beta phase is about 882.degree. C., for pure titanium. Elements which promote higher transformation temperatures are known as alpha-stabilizers. Examples of alpha stabilizers are aluminum and lanthanum. Elements which promote lower transformation temperatures are known as beta-stabilizers. Beta stabilizers are classified in two groups: the isomorphous beta stabilizers, exemplified by molybdenum, niobium, tantalum, vanadium and zirconium; and the eutectoid beta stabilizers, exemplified by cobalt, chromium, iron, manganese and nickel. Thus, alloying elements, there are three general classes of titanium base alloy: alpha, alpha-beta and beta.
An example of a high strength titanium base alloy containing the beta stabilizers vanadium and iron and the alpha stabilizer aluminum is disclosed in U.S. Pat. No. 3,802,877. However, the biocompatibility of this alloy may be compromised because of the presence of vanadium, which should be avoided in an alloy used to fabricate an implant.
Bone implants made from titanium or titanium-containing alloys are known in the art. Implants, such as plates and screws, made from pure titanium were used in 1951 for the fixation of bone fractures when it was found by Jergesen and Leventhal that these implants exhibited good tissue tolerance. See Laing, P. G. "Clinical Experience with Prosthetic Materials," ASTM Special Technical Publication 684 (1979), pp. 203-4. However, although pure titanium has excellent corrosion resistance and tissue tolerance, its relative low strength, when compared to stainless steel, and unfavorable wear properties, limited its use for general bone implants.
In the 1970s pure titanium for surgical implants was replaced by an alloy containing aluminum and vanadium (Ti-6Al-4V) for the manufacture of high strength femoral prostheses. However, although no toxic reaction was reported in patients, the known toxicity of vanadium and the association of aluminum with various neurological disorders has raised considerable doubt about the safety of this alloy.
U.S. Pat. No. 4,040,129 discloses an implant for bone surgery and for dental therapeutics containing defined critical amounts of titanium and/or zirconium and other selected metallic elements including niobium, tantalum, chromium, molybdenum and aluminum. Alloying elements of questionable biocompatibility, such as vanadium, are specifically excluded.
In 1980 a Ti-5Al-2.5Fe alloy was disclosed for surgical implant application and in 1985 a Ti-6Al-7Nb alloy was disclosed for the manufacture of various types of femoral component stem. Each of these alloys contained a relatively high proportion of the suspect alloying element aluminum.
A biocompatible titanium base alloy suitable for bone implants should meet at least the following requirements:
1. Potentially toxic elements, such as vanadium, copper and tin, should be avoided completely.
2. Elements which may have potential toxicological problems, such as chromium, nickel and aluminum should be used only in minimal, acceptable amounts.
3. The alloy should have high corrosion resistance.
4. The alloy should have at least the following desired mechanical properties: low modulus, high strength and good smooth and notched fatigue strength.
5. The alloy should have good workability and ductility.
It has now been found that a bicompatible alloy meeting the desired requirements and, in particular, having a combination of high strength and low modulus desirable for orthopaedics but not possessed by any alloy disclosed in the prior art, may be produced, preferably by double plasma melting, from a carefully balanced formulation of beta stabilizers, alpha stabilizers and titanium.